"So you have a complete set of keys to all the doors and gates in the college?"

"That I do, sir, 'cept when people lose 'em," he said darkly. "The only ones I don't have are to the college strong room. You can only get to it through the subrector's room, as I say, up the tower staircase, and the rector and the bursar alone have a key. It is designed that way so that no one person can get into the strong room without at least one other person present," he added.

"And only you have keys to the other rooms?"

"No, sir-the rector also keeps a complete set to all the rooms in his lodgings, but he doesn't hand those out. Students and Fellows alike must come to me, and only me." He shuffled back to his chair and regarded me with curiosity.

"Does the bursar have a key to the subrector's room?"

"The bursar?" Cobbett looked surprised. "No, sir-he has his key to the strong room, but the subrector must be present to let him up to the tower. It's supposed to guard against theft, you see."

"But what if the subrector should be away, and the bursar needs the strong room?"

"Well, then, he would need to come to me or ask the rector to let him up. Why you so interested in keys, anyways?"

"Oh-I have only been wondering how a stray dog might have got into the grove," I replied, though I was now also wondering how Slythurst had obtained a key to Roger Mercer's private chamber. Had he somehow contrived to steal the spare key from Cobbett's cupboard? And if that was the case, how had the person who first turned over Mercer's room let himself in? Who had a third key, except the rector?

"Ah." The old porter rubbed his stubbly chin. "Well, now-I daresay that was my fault, sir-it must be that I didn't check the Brasenose Lane gate carefully enough last night."

A silence followed; it was clear that the old man was uncomfortable telling a lie that reflected poorly on his competence, and that he was doing so dutifully but reluctantly.

"I find that hard to believe," I said, encouragement in my voice. "For everyone tells me you have served the college man and boy and have never neglected your duty."

A look of gratitude spread across the porter's face; he beckoned me closer. I leaned in; his breath was heavy with stale beer.

"I thank you, sir. I told the rector, I said, sir, you know I will do as you wish but I hope no one will ever believe old Cobbett left any cranny of this college unchecked on his rounds. People here know I do my job well, sir." He puffed out his great barrel chest and fell to a fit of coughing.

"Well, I hope you will not be punished for what is not your fault," I said.

"Thank you, sir, you are kind."

"Tell me, Goodman Cobbett," I said casually, turning to go, "if a man ever wanted to go into the town and return after you lock the main gates, might that be possible?"

The porter's face creased into a broad, gummy smile.

"All things are possible, Doctor Bruno," he said, with a wink. "Perhaps you have heard I sometimes come to certain agreements with the undergraduates regarding the locking of the gates. But you should not need any such arrangements-Fellows and guests may have a key to the main gate."

"Really?" I asked, surprised. "So the Fellows may leave the college by the main gate and enter at whatever hour they please?"

"It is not exactly encouraged," Cobbett said, warily, "but yes, they may. Not many of 'em do, mind-they are all too serious-minded for gadding about the town. It's the students who want to get out and are denied the liberty. But I was a young man once, and I say it does more harm than good to deny young men their pleasures. All work and no play makes Jack a dull boy, sir."

I bent slightly and peered through the little window that opened onto the tower archway. Two students in black gowns passed, leather satchels clutched to their chests.

"Can you see from here everyone who comes in and out at night, then?" I asked.

"As long as I'm awake," said Cobbett, with a husky laugh that quickly turned into another round of coughing.

There was more I wanted to ask, but I sensed my questions were making him suspicious, so I turned to the door.

"Thank you for your help, Cobbett-I must be getting along."

"Doctor Bruno," he called, as I opened the door. I turned back. "Please do not repeat what I said about the grove, will you? As much as it pains me, I must do as the rector instructs and say the blame was mine."

I assured him that I would not mention our conversation. His face slumped with relief.

"I will gladly tell you more of locks and keys another time if you care to know," he added, casually twirling Mercer's keys in his stubby fingers. Then he reached beneath the table and pulled out an earthenware flagon, waving it meaningfully in my direction. "But it is thirsty work, all this jawing. Conversation flows all the better for a bit of refreshment, if you catch my meaning."

I smiled. "I will see what refreshment I can find for when we next converse, Cobbett," I said. "I shall look forward to it."

"And I, Doctor Bruno, and I. Leave the door open, if you'd be so kind."

He reached down and ruffled the dog's fur between its ears. I could hear him chuckling to himself as I left the lodge and stood in front of the high main gate, wondering.

I RETURNED to my chamber, glad to rid myself of the shirt and breeches, now stiff with Roger Mercer's blood, and to take the book out of my breeches, where its corners were digging uncomfortably into my stomach. Clad only in my underhose, oblivious to the chill of the room, I took a tinderbox from the mantelpiece and lit one of the cheap tallow candles with which the room had been provided; the room quickly filled with its acrid smoke as I took Mercer's almanac and opened it, this time at the back. There were several blank pages bound into the covers, and one of these was oddly stiff, the paper slightly warped as if it had got wet and then dried out. I sniffed it closely; here the smell of oranges was most insistent. Carefully, so as not to scorch it, I held the page up close to the candle's flame and watched as, slowly, a series of marks in dark brown began to grow visible. Moving the paper up and down past the flame, it gradually revealed its secret writing: a sequence of letters and symbols, with no logical pattern I could discern. Below this was a shorter series of the same symbols, though in a different order: grouped in two lots of three different symbols, then a group of five. It was evidently some kind of cipher, though I knew little of cryptography and had no idea how to begin decoding it. I wondered if Sidney might have a better idea, given that he had had more contact than I with such work, so I took a piece of paper and a quill and made a copy of the symbols exactly as they appeared on the page, thinking I would give this to him to work on. But as I copied the first three lines, it became clear that the symbols were arranged in a sequence of twenty-four, and that this sequence was repeated three times.

I paused. There were twenty-four letters in the English alphabet, but surely no cipher could be that obvious? Nonetheless, I thought it worth a try, and on my copy I wrote out the alphabet underneath the first sequence of twenty-four symbols. If this was a basic substitution cipher, then according to this system the groups of letters underneath might mean something. I copied out the first group of three symbols according to the alphabetical substitution, and as I saw the result, O-R-A, I felt my pulse quicken. Hurriedly I translated the remaining letters of the short phrase, and drew my breath in sharply. I had written the words Ora pro nobis.

Folding the copy carefully and hiding it under my pillow, I laid my head down gratefully, trying to imagine why Roger Mercer had written those words-the refrain from the Catholic Litany of the Saints-invisibly in the back of his almanac. But I had to put the puzzle from my mind; there were more pressing matters for my attention. I had intended only to close my eyes for a few moments before gathering my thoughts and setting them to concentrate on the evening's disputation, which was supposed to be the crowning glory of my first visit to Oxford, but I was awakened all of a sudden by a furious hammering on the door and sat upright, confused and bleary.